Bridge circuits are commonly used to deliver an AC voltage waveform to an electrical load. A common configuration for such a half bridge inverter circuit is shown in FIG. 1. Half bridge 10 includes two switches 11 and 12, the switches each including a transistor 2 and 4, respectively, such as an IGBT or MOSFET, connected anti-parallel to a diode 3 and 5, respectively, as shown. A pulsed-wave modulated (PWM) voltage source is used to control the gate voltage of each of the transistors of the switches 11 and 12, resulting in a switching waveform at the output 6 of the half bridge. A filter 20, which includes an inductive element 21 and a capacitive element 22, is then used to filter the switching waveform, resulting in the desirable low-frequency voltage or current waveform being delivered to the electrical load 15 at the output. The electrical load 15 is connected to the inverter circuit by an electrical connector 14, such as a cable, which in FIG. 1 is represented by inductor 16 and capacitors 17 and 18, connected as shown.
In applications in which the electrical load 15 has a substantial inductance, and therefore a substantial reactance, for example when the electrical load is a motor, the filter 20 can be omitted, and the reactance of the electrical load 15 can be used to filter the switching waveform. However, this technique has several disadvantages. Parasitic resonances in the cable 14 and motor 15 can be excited by the switching waveform, resulting in high-voltage spikes which damage the motor 15 and require over-specifying the voltage withstand of various components. Additionally, any capacitance in the cable 14 or motor 15 will be charged and discharged at the switching rate, which represents lost energy. Also, current spikes associated with charging and discharging the motor and cable capacitance constitute current dipoles or current loops which become emitters of electromagnetic interference.
Inserting the filter 20 between the half bridge 10 and the cable 14 can eliminate the problems described above. However, the filter 20 must have a sufficiently low cut-off frequency to effectively attenuate the switching waveform. For conventional switches 11 and 12 used in the half bridge 10, the switching frequency is typically about 12 kHz, and is limited to no more than about 50 kHz, which is approximately the maximum frequency at which conventional switches 11 and 12 can be effectively switched without incurring unacceptably high switching losses. Consequently, the maximum frequency at which the fractional admittance of the filter is too high to prevent substantial output ripple at the switching frequency, i.e., the cut-off frequency of the filter, needs to be about 10 kHz or less, corresponding to a 3 dB roll-off frequency of about 1 kHz for the filter. The frequency response of the filter is illustrated in FIG. 2. As seen in the plot of the fractional admittance of the filter as a function of frequency, the fractional admittance drops substantially below 1 at a frequency close to 1 kHz and approaches a small value at a frequency of just above 10 kHz. Such a filter requires excessively large inductors and/or capacitors, resulting in a substantial increase in cost for the circuit. Furthermore, the half bridge inverter is limited to applications in which the frequency of the signal delivered to the electrical load 15 is much less than 1 kHz. There are many applications in which smaller, more compact inverter circuits are desirable.